1. Field of the Invention
This invention relates to a semiconductor laser amplifier, and more particularly to a semiconductor laser amplifier used in optical communications to amplify optical signals.
2. Description of the Related Art
Since an expensive optical repeater may be replace by only one semiconductor laser amplifier, semiconductor laser amplifiers are expected to significantly reduce the cost of building optical fiber relay networks in the future.
FIG. 1 is a block diagram of a conventional typical semiconductor laser amplifier.
As shown in FIG. 1, the light propagated along the optical fiber 10 is supplied via the image forming lens 12 to the optical waveguide layer 16 of the semiconductor laser amplifier 14. The optical waveguide layer 16 also serves as an optical active layer, in which light is amplified and leaves the amplifier 14. The amplified light from the amplifier 14 is supplied via the image forming lens 18 to the optical fiber 20, returning to the optical fiber relay network.
To achieve a sufficient gain at the amplifier 14, it is necessary to make the length of the optical waveguide layer 16 or a cavity length L sufficiently long. For this reason, semiconductor laser amplifiers must have a laser chip length of approximately 400 to 1000 .mu.m, more than twice an ordinary laser chip length of 200 to 300 .mu.m. This leads to a poorer productivity.
Further, in the optical system, it is necessary to connect lenses or fibers to both ends of a chip of the order of 400 to 1000 .mu.m in length, which makes it difficult not only to design and fabricate amplifiers but also to perform important work, such as hermetic sealing, in terms of reliability.
A semiconductor laser amplifier proposed especially to overcome a poor productivity is shown in FIG. 2.
As shown in FIG. 2, the light propagated along the optical fiber 10 is supplied via the beam splitter 30 to the round-tip optical fiber 32. The light leaving the round-tip optical fiber 32 is supplied to the optical wave guide layer 16 of the semiconductor laser amplifier 14. The light travels along the optical waveguide layer 16 increasing its gain until it reaches the end of the resonator. Formed on the end is a high reflection coating film (HR film) 34, which reflects the light. The reflected light then travels backward along the optical waveguide layer 16 further increasing its gain. The light with the increased gain is supplied again to the round-tip optical fiber 32, then split at the beam splitter 30 to go to the optical fiber 20, and returns to the optical fiber relay network.
By propagating light forward and then backward along the optical waveguide layer 16 of the semiconductor laser amplifier 14, it is possible to achieve nearly twice the length of the optical waveguide layer 16, that is, approximately 2L with respect to the cavity length of L, which provides a small chip with a large gain.
However, because the incoming light and outgoing light are propagated along the same optical waveguide layer, the beam splitter 30 to sort them must be added in the optical fiber. Presently, it is extremely difficult to completely separate the incoming light from the outgoing light using the beam splitter 30, which creates problems. For example, leakage of optical components is liable to make signal noises. Some of the light reflected at the beam splitter 30 can return to the optical waveguide layer 16, creating a signal noise. There is also a reliability problem: it has not been verified yet that the beam splitter 30 ensures the sufficiently stable properties against changes in ambient temperatures and aging.
To increase the coupling efficiency of the round-tip fiber 32, it is necessary to adjust the position with a high accuracy on the submicron order, which makes it difficult to fabricate the optical system. Particularly, in hermetically sealing the amplifier 14 in a package to fabricate a module, the round-tip fiber is soldered or bonded with an adhesive, which often causes misalignment during work, resulting in a poor fabrication yield.
As noted above, in conventional semiconductor laser amplifiers, the length of elements (cavity length) is large, leading to a poor productivity.
Some semiconductor laser amplifiers are available which have an improved productivity, but the incoming light and outgoing light are still propagated along the same optical fiber, which involves many factors creating signal noises. Use of insufficiently stable parts in terms of a change in ambient temperature and aging impairs the reliability. In addition, use of a round-tip fiber necessitates submicron adjustment in fabricating the module, making the work difficult.